Bisphenol A [2,2-bis(4-hydroxyphenyl)propane] is usually prepared by reacting phenol with acetone in the presence of a homogeneous acid or a solid acid catalyst. The phenol, water and other by-products formed by the reaction, in addition to bisphenol A. The main component of the by-products is 2-(2-hydroxyphenyl)-2-(4-hydroxyphenyl)propane (hereinafter, referred to as o,p′-BPA), and in addition, it includes trisphenol, a polyphenol compound, a chroman compound and colored impurities.
Examples of a homogeneous acid to be used as a catalyst, includes hydrochloric acid and sulfuric acid. In the case where the homogeneous acid is used, since it is possible to proceed the reaction while precipitating crystals of an adduct of phenol with bisphenol A by reacting them at lower temperatures, bisphenol A can be prepared with a high conversion of acetone and a high selectivity by decreasing the amount of the by-produced o,p′-BPA as an isomer thereof. However, the homogeneous acid such as hydrochloric acid requires removal of the catalyst from a reaction mixture or neutralization of the reaction mixture, and thus the process becomes complicated. Homogeneous dissolution of the acid in the reaction solution further causes corrosion of apparatus used in the reaction. Therefore, the reaction apparatus vessels should be made of expensive, anti-corrosive materials, thus being uneconomical.
As a solid acid catalyst, a sulfonic acid-type cation-exchange resin is usually used. The reaction for preparing bisphenol A essentially proceeds only with an acid catalyst, but if such a solid acid catalyst is used, the process in which acetone diffuses from the surface of the catalyst particles to an active site on the catalyst is involved, and thus the reaction rate is lowered. Thus, generally a method for improving the catalytic activity and the selectivity comprises allowing a compound containing a mercapto group to coexist in the reaction system. Specifically, there may be mentioned a method for passing a free type of a compound containing a mercapto group, such as alkylmercaptan together with phenyl and acetone as raw materials, through a fixed bed reactor filled with a sulfonic acid-type cation-exchange resin (Japanese Examined Patent Application Publication No. 45-10337, U.S. Pat. No. 6,414,200), a method for covalently binding a part of sulfonic acid groups of the sulfonic acid-type cation-exchange resin with a compound containing a mercapto group, and a method for ionically binding a part of sulfonic acid groups in a sulfonic acid-type cation-exchange resin with a compound containing a mercapto group (Japanese Examined Patent Application Publication No. 46-19953). In the method for passing a free type of a compound containing a mercapto group, such as alkylmercaptan together with phenol and acetone as raw materials, through the fixed bed reactor filled with a sulfonic acid-type cation-exchange resin, since a certain amount of a compound containing a mercapto group is usually allowed to exist in the reaction system, the compound containing a mercapto group may cause the coloration of bisphenol A, although it is advantageous in that deterioration of the catalyst is small. Thus, the compound containing a mercapto group should be removed and recovered. On the other hand, in the method for binding a part of the sulfonic acid groups in a sulfonic acid-type cation-exchange resin with a compound containing a mercapto group, it is advantageous in that the loss of the compound containing a mercapto group is small, and thus it is not necessary to recover the compound containing a mercapto group, as compared to the method wherein a free type of a compound containing a mercapto group exists in the reaction system. In particular, Japanese Unexamined Patent Application Publication Nos. 08-187436, 08-089819 and 10-211433 describe that a sufficient reaction rate of acetone is obtained by modifying the structure of a compound containing a mercapto group which binds to a strongly acidic ion-exchange resin.
On the other hand, for example, it has also been reported that the activity of a sulfonic acid-type cation-exchange resin as an acid catalyst is improved, which has been lower than that of the previously described homogeneous acid. First, in the case where a particle diameter of a sulfonic acid-type cation-exchange resin to be used is large, since the reaction raw materials do not sufficiently diffuse within the particles, a sufficient conversion of acetone is not obtained. Thus, Japanese Unexamined Patent Application Publication No. 62-178532 proposes to use the fine particles having an effective diameter of 0.3 mm or less, or the fine-powdered sulfonic acid-type cation-exchange resin. Further, Japanese Unexamined Patent Application Publication No. 6-340563 likewise provides a particle diameter of a sulfonic acid-type cation-exchange resin to be used and the distribution degree of the particle diameter and specifies a more preferred range thereof. Further, Japanese Unexamined Patent Application Publication Nos. 4-268316 and 2002-253971 describe a method for forming a sulfonic acid-type cation-exchange resin product having a desired particle diameter. As such, the particle diameter of a sulfonic acid-type cation-exchange resin is an important factor in obtaining a sufficient reaction conversion.
In addition, various kinds of improvements on the structure of a resin product which is a base material of a sulfonic acid-type cation-exchange resin have been made. The sulfonic acid-type cation-exchange resin is a resin obtained by sulfonating a styrene-divinylbenzene copolymer which is obtained by radically copolymerizing styrene and divinylbenzene. The divinylbenzene in polymerization does not only prevent a polystyrene chain from dissolving in an organic solvent, but also it is an important factor in controlling the size of a pore (a gel micropore) of the sulfonic acid-type cation-exchange resin formed by capturing a polar solvent by its content, or the mechanical strength of the sulfonic acid-type cation-exchange resin. In other words, a sulfonic acid-type cation-exchange resin with a low content of divinylbenzene has a high catalytic activity due to a large gel micropore, but the mechanical strength is low. In addition, in the case where the content thereof is high, the mechanical strength increases, but the gel micropore size decreases, which causes decreased activity. For an ion-exchange resin of which the degree of crosslinking becomes increased by increasing the content of divinylbenzene, there may be mentioned one which forms a large hole having a size of 20 nm or more, referred to as a “macropore” within the particles by a physical treatment in order to improve the diffusion within the particles.
However, in the case where an ion-exchange resin having macropores adsorbs molecules having high polarity, such as water, a crosslinked structure tends to inhibit the bulge of particles caused by the swelling, which eventually collapses when it can no longer endure the pressure from the swelling. Japanese Unexamined Patent Application Publication Nos. 5-97741 and 6-320009 describes a method which complements the respective defects by simultaneously filling a sulfonic acid-type cation-exchange resin having a low content of divinylbenzene and a sulfonic acid-type cation-exchange resin having a high content of divinylbenzene into a reactor. Further, WO 2000/00454 (Nippon Steel Chemical Co., Ltd.) reports an improvement on a reaction conversion, suggesting a sulfonic acid-type cation-exchange resin having large gel micropores by using large molecules such as divinylbiphenyl instead of divinylbenzene.
As such, various technologies have been examined for the catalyst, but none has been satisfactory with respect to the selectivity. Even when the activity decreases slightly, if it is possible to develop a catalyst having a high selectivity, the loading of the a by-product recovering process can be reduced in the preparation process and a phenol/acetone ratio as raw materials can be reduced by elevating the reaction temperature without deteriorating the selectivity, and thus the cost for recovering the phenol can be largely reduced. Here, the reduced level of the activity can be compensated by increasing the reactor size, wherein the increase in cost in the preparation of bisphenol is extremely small. Further, in view of the industrial applications, it is desirable to improve the life of catalysts. Therefore, the development of a catalyst having a high selectivity and a long life time is demanded.    [Patent Document 1] U.S. Pat. No. 6,414,200    [Patent Document 2] Japanese Unexamined Patent Application Publication No. 08-089819